1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
In recent years, it is common for color image forming apparatuses to have a function of automatically controlling image density, since there is a demand for accurate color reproducibility and tone stability. In image density control, generally, a plurality of test toner images (patches) that have been formed on an image carrier (rotation member) while changing image creation conditions are detected using an image density detector arranged in the image forming apparatus, and an optimal image creation condition is determined based on the conversion results.
At this time, with regard to toner image detection, the reflectance on the surface of the image carrier is different depending on the position on the image carrier. Therefore, in order to accurately detect density, it is necessary to obtain output indicating toner presence/absence at the same position on the image carrier, and normalize the output indicating toner presence by the output indicating toner absence. On the other hand, the circumference of the image carrier changes due to variation in components, atmosphere/environment of the image forming apparatus, and the like, and errors will occur in specifying positions on the image carrier if the circumference is treated as a fixed value. In view of this, it is necessary to dynamically measure information with regard to the circumference of the image carrier.
Japanese Patent Laid-Open No. 10-288880 proposes a method for measuring the circumference of an image carrier in an image forming apparatus that adopts an intermediate transfer method by attaching a mark to the surface of an edge portion of an intermediate transfer member (rotation member), and receiving reflected light from the mark using an optical sensor.
However, the conventional technology has problems described below. For example, in the image forming apparatus that adopts an intermediate transfer method disclosed in Japanese Patent Laid-Open No. 10-288880, the intermediate transfer member needs to be rotated to the position at which the mark is set and further rotated one full rotation from that position. Specifically, this is because the mark may not necessarily be positioned very close to the optical sensor when circumference measurement is started. In a worst-case scenario, the intermediate transfer member may need to be rotated approximately two full rotations before the circumference can be detected. In image density control, if time is spent on circumference measurement, a period (so-called downtime) in which image formation cannot be executed also increases, which impairs usability.